Method for producing ethylene homo-and copolymers by intensively mixing a reactive reaction component with a mobile flow medium

ABSTRACT

The invention relates to a method for producing ethylene homo- and copolymers in a tubular reactor at pressures above 1000 bar and temperatures in the range from 120 to 350° C. by free-radical polymerization, in which firstly small amounts of free-radical chain initiators are added to a mobile flow medium comprising ethylene, molecular weight regulator and optionally polyethylene, and the polymerization is then carried out. In accordance with the invention, the mobile flow medium is firstly split into two volume elements flowing separately from one another, the volume elements flowing separately from one another are then set in rotation in opposite directions by means of suitable flow elements, the flowing volume elements rotating in opposite directions are then recombined to form a mobile flow medium, and then, at the moment of or shortly after the combining of the flowing volume elements rotating in opposite directions, the free-radical chain initiator is fed into the sheared interfacial region between the flowing volume elements rotating in opposite directions. The invention also relates to an apparatus for carrying out this method.

The present invention relates to a method for producing ethylene homo-and copolymers in a tubular reactor at pressures above 1000 bar andtemperatures in the range from 120 to 350° C. by free-radicalpolymerization, in which firstly small amounts of free-radical chaininitiators are fed to a mobile flow medium comprising ethylene,molecular weight regulator and optionally polyethylene, and thepolymerization is then carried out.

The high-pressure polymerization process is a proven process for theproduction of low density polyethylene (LDPE) which is carried outhighly successfully on a large industrial scale in numerous plantsworldwide. The polymerization in high-pressure polymerization is usuallyinitiated by atmospheric oxygen, by peroxides, by other free-radicalformers or by mixtures of these. In practice, it has proven particularlyadvantageous to initiate the polymerization reaction “simultaneously” ata plurality of points within the reactor and thus to keep the reactoryield high and the product quality at a uniformly high level. To thisend, the free-radical chain initiators employed for initiation of thepolymerization have to be added to the reaction medium in a suitablemanner.

The effectiveness of a selected free-radical chain initiator depends onhow rapidly it is mixed with the initially introduced reaction medium inthe individual case. To this end, so-called injection fingers are usedin large-scale industrial plants in the production of high-pressurepolyethylene. EP-A-0 449 092 describes how free-radical chaininitiators, also referred to as initiators below, initiator mixtures orsolutions of initiators in organic solvents, are metered in at aplurality of points along a reactor via injection fingers.

An improvement in the mixing of the metered-in initiator andconsequently an improvement in the product quality has also beenachieved by increasing the flow velocity in the mixing zones. U.S. Pat.Nos. 4,135,044 and 4,175,169 describe how products having very goodoptical properties can be produced in high yields and with a relativelylow pressure drop over the length of the reactor by means ofcomparatively small tube diameters in the initiation and reaction zonesof a high-pressure reactor, relative to the enlarged tube diameter inthe cooling zone.

Finally, U.S. Pat. No. 3,405,115 describes the particular importance ofuniform initiation of the polymerization reaction and of optimum mixingof the reaction components for the quality of the polyethylene, for ahigh reactor yield and for establishing uniform reactor operation. Tothis end, initiators are mixed with sub-streams of cold ethylene in aspecial mixing chamber and only thereafter fed to the actual reactor. Inthe mixing chamber, the fluid, in which the initiator does not decomposeowing to the low temperature prevailing therein, is re-directed a numberof times and passed through channels.

A common feature of all known methods and apparatuses for feedingfree-radical chain initiators to the reaction mixture is that the rateand intensity of the mixing process are still unsatisfactory.

It was therefore an object of the present invention to indicate a methodby which the high-pressure polymerization of ethylene in tubularreactors can be carried out with improved reactor yields, based on theadded amount of free-radical chain initiator, and with improved productquality of the resultant polyethylene by increasing and intensifying therate and intensity of mixing of the free-radical chain initiator withthe mobile flow medium at the moment of feeding.

This object is achieved by a method of the generic type mentioned at theoutset whose characterizing features are to be regarded as being thatthe mobile flow medium is firstly split into two volume elements flowingseparately from one another, in that the volume elements flowingseparately from one another are then set in rotation in oppositedirections by means of suitable flow elements, in that the flowingvolume elements rotating in opposite directions are then recombined toform a mobile flow medium, and in that, at the moment of or shortlyafter the combining of the flowing volume elements rotating in oppositedirections, the free-radical chain initiator is fed into the shearedinterfacial region between the flowing volume units rotating in oppositedirections.

In an embodiment of the method which is preferred in accordance with theinvention, the splitting of the mobile flow medium is carried out insuch a way as to form a core stream and volume elements flowing close tothe wall.

Molecular weight regulators which can be employed in accordance with theinvention are customary polar or nonpolar organic compounds, such asketones, aldehydes, alkanes or alkenes having from 3 to 20 carbon atoms.Preferred molecular weight regulators are acetone, methyl ethyl ketone,propionaldehyde, propane, propene, butane, butene or hexene.

Free-radical chain initiators which can be used in accordance with theinvention are peroxides, such as aliphatic diacyl (C₃ to C₁₂)peroxides,dialkyl (C₃ to C₁₂)peroxides or peroxyesters, tertiary-butylperoxy-pivalate (TBPP), tertiary-butyl peroxy-3,5,5-trimethyl-hexanoate(TBPIN), di-tertiary-butyl peroxide (DTBP) or mixtures or solutions ofthese in suitable solvents. The free-radical chain initiators are, inaccordance with the invention, introduced in amounts in the range from10 to 1000 g/t of PE produced, preferably from 100 to 600 g/t of PEproduced.

The mobile flow medium to which the above-mentioned free-radical chaininitiators are fed in accordance with the invention may, besidesethylene, additionally comprise, as comonomer, 1-olefins having from 3to 20 carbon atoms, preferably having from 3 to 10 carbon atoms, in anamount in the range from 0 to 10% by weight, based on the amount ofethylene monomer, preferably in an amount in the range from 1 to 5% byweight. In addition, the mobile flow medium may, in accordance with theinvention, comprise polyethylene in an amount in the range from 0 to 40%by weight, based on the total weight of the monomers, preferably from 0to 30% by weight.

In a particularly preferred variant of the method according to theinvention, the free-radical chain initiators are introduced in a regionof the tubular reactor in which the flow velocity of the mobile flowmedium has been increased to between 1.2 and 2.8 times, preferably tobetween 1.8 and 2.5 times, the flow velocity within the feed zone of thetubular reactor through a reduction in the diameter of the tubularreactor to a value of from about 0.6 to 0.9 times the diameter D of thereactor in the feed zone. Expressed in absolute figures, the flowvelocity of the mobile flow medium in the feed zone of the free-radicalinitiators is, in accordance with the invention, in the range from 10 to40 m/s, preferably from 15 to 30 m/s, particularly preferably from 20 to25 m/s.

The method according to the invention enables the amount of free-radicalchain initiator added to be significantly reduced for the same amount ofLDPE produced, and consequently enables the high-pressure polymerizationto be carried out more economically.

Furthermore, the LDPE prepared by the method according to the inventionhas improved optical properties owing to smaller high-molecular-weightfractions having a molar mass of greater than 10⁶ g per mole.

In addition, the method according to the invention has the advantagethat more stable reactor operation can be maintained at unusually highmaximum temperatures of up to 350° C. without a tendency towarddecomposition occurring.

A further advantage of the method according to the invention is to beregarded as the fact that the polymerization is initiated at lowertemperatures and that the temperature increase of the reaction mixturethen takes place in a more controlled manner. The life of thefree-radical initiators, which usually have only a relatively short halflife, is better utilized for the polymerization and thus for theproduction of LDPE.

The invention also relates to an apparatus for carrying out the method,comprising a tubular reactor section having an internal diameter D and alength in the range from 30 to 50·D, preferably from 35 to 45·D, and oneor more feed nozzles for the free-radical chain initiator, whosecharacterizing features are to be regarded as being that separatingelements for separating the mobile flow medium into volume elementsflowing separately from one another are arranged in the interior of thetubular reactor over a length in the range from 2 to 6·D, in that inaddition at least one flow element which is capable of setting a flowmedium flowing along it in rotation is arranged in the region of theseparating elements, and in that one or more feed nozzles for thefree-radical chain initiator are arranged downstream of the separatingelements and flow elements.

The separating element for the separation of the mobile flow medium is,in a preferred embodiment of the apparatus according to the invention,an internal tube having a diameter in the range from 0.5 to 0.7·D withwhich the mobile flow medium is separated into a core stream in theinterior of the internal tube and a shell stream outside the internaltube, but inside the tubular reactor.

Flow elements which, in their geometrical form, viewed in thelongitudinal direction, represent plates twisted by an angle of ±α arepreferably arranged inside the internal tube and outside the internaltube, with the flow elements inside the internal tube and outside theinternal tube being twisted in the same direction. The maximum length ofthe flow elements corresponds to the length of the internal tube, butthe flow elements may also have smaller dimensions, where the angle α bywhich the plates are twisted should be at least 90°, but can just aswell also be selected at a larger value in order to increase therotation of the flowing volume element.

The feed nozzles, of which at least one, but preferably a plurality, arearranged, in accordance with the invention, in the flow direction,viewed at the end of the internal tube, have exit bores of at most 1 mm,preferably at most 0.7 mm, particularly preferably at most 0.5 mm. Theseparation of the feed nozzles from the end of the internal tube shouldbe at most 1·D, preferably at most 0.5·D.

A conical transition piece, in the region of which the internal diameterof the tubular reactor is reduced from D to between about 0.9 and 0.6·D,is preferably located upstream of the separating elements for separatingof the mobile flow medium into volume elements flowing separately fromone another or downstream of the feed nozzle(s). The conical transitionpiece is located at a separation of at most 1·D, preferably at most0.5·D, from the feed nozzle(s) or the separating elements and has alength in the range from 3 to 7·D, preferably in the range from 4 to6·D.

In the course of the conical transition piece, the flow velocity of themobile flow medium increases to between about 1.2 and 2.8 times,preferably to between 1.8 and 2.5 times, the flow velocity within thefeed zone of the tubular reactor.

If the conical transition piece is arranged downstream of the feednozzle(s), the actual reaction tube, which has a length in the rangefrom 15 to 30·D, preferably from 20 to 27·D, and an internal diameterwhich corresponds to the internal diameter of the conical end part ofthe conical transition piece, is connected downstream of the conicaltransition piece.

In the course of the reaction tube, the high flow velocity is thereforemaintained, ensuring that the mixing of the reaction components and thefree-radical chain initiators within the mobile flow medium is virtuallycomplete. After passing through the reaction tube at a high flowvelocity, the flow velocity of the mobile flow medium can therefore bereduced again, which can take place in a second conical transition piecehaving a length which essentially corresponds to the length of the firstconical transition piece.

If the conical transition piece is arranged upstream of the separatingelements for separating the mobile flow medium into volume elementsflowing separately from one another, the addition in accordance with theinvention of free-radical chain initiators takes place in the front partof the reaction tube itself, in which an increased flow velocity alreadyprevails. Although the reduced internal diameter of the reaction tube tobetween 0.9 and 0.6 D necessitates a reduction in the geometry of theseparating elements and likewise of the flow elements, this arrangementmay, however, in some cases have advantages in the metering offree-radical chain initiators and the initiation of the polymerizationreaction, due to the higher flow velocity.

The lengths and diameters of the individual parts of the apparatusaccording to the invention can be varied in broad ranges, influencingthe mixing quality and also the pressure drop within the reactionmixture. Furthermore, feed nozzles, injection fingers or injectionnozzles of various designs can be combined with the apparatus accordingto the invention. The flow velocity of the mobile flow medium can,through variation of the mass flow velocity, adopt values in the rangebetween 10 m/s and 40 m/s, preferably between 15 m/s and 30 m/s,particularly preferably between 20 m/s and 25 m/s.

The invention will be described more clearly below for the personskilled in the art by means of a drawing, without being restricted tothe embodiment illustrated therein.

THE BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a side view of a vertical section through an apparatusaccording to the invention.

Reference symbols show the tubular reactor 1, which has a diameter D inthe range from 20 to 100 mm. An internal tube 2, which, in the apparatusdepicted, has a diameter of 0.6·D and a length of 4·D, is arranged inthe feed zone of the tubular reactor 1. A flow element 3, which has theshape of a plate twisted by an angle of ±90°, is arranged inside theinternal tube 2. Further flow elements 4, 4′, each of which has theshape of a plate twisted by an angle of −90°, are arranged outside theinternal tube 2, but still inside the tubular reactor 1. A feed nozzle5, with the aid of which the free-radical chain initiator is fed intothe sheared interfacial region of the flowing volume elements rotatingin opposite directions, is arranged at a separation of 0.5·D from theend of the internal tube 2. The feed nozzle 5 has an exit bore with adiameter of 0.5 mm, which is shown in the detail enlargement. Theconical transition part 6, by means of which the internal diameter ofthe tubular reactor is reduced to a value of 0.72·D, follows in the flowdirection S at a separation of 0.5·D from the feed nozzle 5. The conicaltransition piece in the representation in FIG. 1 has a length of 5·D.The conical transition piece 6 is followed in the flow direction S bythe reaction zone 7, which has a length, not shown in full, of 25·D, andwhich is followed by the second conical transition piece 8, likewise notshown in full, by means of which the internal diameter of the tubularreactor 1 is increased back to a value of D.

After the invention has now been explained in many details by thedrawing, the following is intended to demonstrate the technicaladvantages of the invention more clearly to the person skilled in theart with reference to working examples.

EXAMPLE 1 COMPARATIVE EXAMPLE

The polymerization of ethylene was carried out in a tubular reactorhaving an internal diameter D of 39 mm at a throughput of 30 t/h under apressure of 2900 bar. The free-radical chain initiator employed was amixture of TBPP, TEPIN and DTBP. The molecular weight regulator employedwas propionaldehyde in an amount of 0.048% by weight, based on the totalweight of ethylene. The amount of free-radical initiators employed andthe results of the polymerization are shown in the table at the end ofthe working examples.

EXAMPLES 2 AND 3 ACCORDING TO THE INVENTION

The polymerization of ethylene was carried out under the same conditionsas in Example 1 using a mixing apparatus depicted in FIG. 1. The amountof free-radical initiators employed and the results of thepolymerization are shown in the table at the end of the workingexamples.

EXAMPLE 4 COMPARATIVE EXAMPLE

The polymerization of ethylene was carried out in a tubular reactorhaving an internal diameter D of 39 mm at a throughput of 30 t/h under apressure of 3000 bar. The free-radical chain initiator employed wasagain a mixture of TBPP, TEPIN and DTBP. The molecular weight regulatoremployed was propionaldehyde in an amount of 0.039% by weight, based onthe total weight of ethylene. The amount of free-radical initiatorsemployed and the results of the polymerization are shown in the table atthe end of the working examples.

EXAMPLES 5 AND 6 ACCORDING TO THE INVENTION

The polymerization of ethylene was carried out under the same conditionsas in Example 4 using a mixing apparatus depicted in FIG. 1. The amountof free-radical initiators employed and the results of thepolymerization are shown in the following table.

Experimental parameters and results

TBPP TBPIN DTBP Scatter MFI Exp. consump. consump. consump. value HazeDensity [dg/10 Conversion No. [g/t of PE] [g/t of PE] [g/t of PE] [%][%] [kg/m³] min] [%] 1 111 157 162 21.0 8.1 923.2 0.83 27.3 2 95 148 15218.3 7.2 923.7 0.81 27.9 3 92 140 144 17.2 7.4 923.6 0.89 27.5 4 275 31622 14.1 7.2 926.8 0.29 24.1 5 220 302 21 11.0 5.6 927.2 0.31 24.0 6 211345 24 11.2 6.0 926.8 0.27 24.5

The product parameters shown in the above table were determined by thefollowing measurement methods:

Scatter value: no standard

Haze: in accordance with ASTM D 1003

Density: in accordance with ISO 1183

MFI: as MFI _((190/2.16))[dg/min] in accordance with DIN 53735

Conversion: production [t/h]/ethylene throughput [t/h]

It is clear from the examples that the method according to the inventionsignificantly improves the conversion and especially the productproperties. It was possible to reduce the amount of free-radical chaininitiators employed by about 20% in accordance with the invention, andthe operating constancy of the tubular reactor was increased.

What is claimed is:
 1. A method for producing ethylene homo- andcopolymers in a tubular reactor at pressures above 1000 bar andtemperatures in the range from 120 to 350° C. by free-radicalpolymerization, which comprises adding a free-radical chain initiator toa mobile flow medium comprising ethylene, molecular weight regulator andoptionally polyethylene, and the polymerization is then carried out, andthe mobile flow medium is firstly split into two volume elements flowingseparately from one another, in that the volume elements flowingseparately from one another are then set in rotation in oppositedirections by means of flow elements, in that the flowing volumeelements rotating in opposite directions are then recombined to form amobile flow medium, and in that, at the moment of or shortly after thecombining of the flowing volume elements rotating in oppositedirections, the free-radical chain initiator is fed into the shearedinterfacial region between the flowing volume units rotating in oppositedirections.
 2. The method according to claim 1, wherein the mobile flowmedium is split in such a way that a core stream and a shell stream areformed.
 3. The method according to claim 1, wherein the molecular weightregulator is a polar or nonpolar organic compound.
 4. The methodaccording to claim 1, wherein the free-radical chain initiator is atleast one peroxide, or solution of said peroxide in a solvent in amountsin the range from 10 to 1000 g/t of polyethylene produced.
 5. The methodaccording to claim 1, wherein, besides ethylene, the mobile flow mediumto which the free-radical chain initiator is fed additionally comprises,as the comonomer, a 1-olefin having from 3 to 20 carbon atoms, in anamount in the range from 0 to 10% by weight, based on the amount ofethylene monomer.
 6. The method according to claim 1, wherein the mobileflow medium additionally comprises polyethylene in an amount in therange from 0 to 40% by weight, based on the total weight of themonomers.
 7. The method according to claim 1, wherein the free-radicalchain initiator is introduced in a region of the tubular reactor inwhich the flow velocity of the mobile flow medium has been increased tobetween 1.2 and 2.8 times, the flow velocity within the feed zone of thetubular reactor by reducing the diameter of the tubular reactor to avalue of between about 0.6 and 0.9 times the diameter D of the reactorin the feed zone.
 8. An apparatus for carrying out the method accordingto claim 1, comprising a tubular reactor section having an internaldiameter D and a length in the range from 30 to 50·D, and one or morefeed nozzles for the free-radical chain initiator, separating elementsfor separating the mobile flow medium into volume elements flowingseparately from one another are arranged in the interior of the tubularreactor over a length in the range from 2 to 6·D, in that in addition atleast one flow element which sets a flow medium flowing along saidlength in rotation is arranged in the region of the separating elements,and in that one or more feed nozzles for the free-radical chaininitiator are arranged downstream of the separating elements and flowelements.
 9. The apparatus according to claim 8, wherein the separatingelement for separating the mobile flow medium in an internal tube havinga diameter in the range from 0.5 to 0.7·D, by means of which the mobileflow medium is separated into a core stream in the interior of theinternal tube and into a shell stream outside the internal tube, butinside the tubular reactor.
 10. The apparatus according to claim 8,wherein the flow elements which, in their geometrical form, viewed inthe longitudinal direction, represent plates twisted by an angle of ±αare arranged inside the internal tube and outside the internal tube,with the flow elements inside the internal tube and outside the internaltube being twisted in opposite directions.
 11. The apparatus accordingto claim 10, wherein the maximum length of the flow elements correspondsto the length of the internal tube, and in that the angle a by which thesheets are twisted is at least 90°.
 12. The apparatus according to claim8, wherein the feed nozzle, of which at least one, are arranged at theend of the internal tube, viewed in the flow direction, have exit boresof at most 1 mm.
 13. The apparatus according to claim 8, wherein aconical transition piece, in the region of which the internal diameterof the tubular reactor is reduced from D to between about 0.9 and 0.6·D,is arranged upstream of the separating elements for separating themobile flow medium into volume elements flowing separately from oneanother or downstream of the feed nozzle(s), and in that the conicaltransition piece is arranged at a separation of at most 1·D, from theseparating elements or the feed nozzle(s) and has a length in the rangefrom 3 to 7·D.
 14. The apparatus according to claim 8, wherein thereaction tube, which has a length in the range from 15 to 30·D, and hasan internal diameter which corresponds to the internal diameter of theconical end part of the conical transition piece, is arranged downstreamof the conical transition piece.
 15. The method according to claim 3,wherein said molecular weight regulator is a ketone, aldehyde, alkane oralkene having from 3 to 20 carbon atoms.
 16. The method according toclaim 15, wherein the molecular weight regulator is acetone, methylethyl ketone, propionaldehyde, propane, propene, butane, butene orhexene.
 17. The method according to claim 4, wherein the free-radicalchain initiator is tertiary-butyl peroxypivalate, tertiary-butylperoxy-3,5,5-trimethyl-hexanoate, di-tertiary-butyl peroxide or mixturesor solutions of said peroxides in amounts in the range from 100 to 600g/t of polyethylene.
 18. The method according to claim 5, wherein the1-olefin has from 4 to 10 carbon atoms in an amount in the range from 1to 5% by weight.
 19. The method as claimed in claim 6, wherein themobile flow medium additionally comprises polyethylene in an amount upto 30% by weight, based on the total weight of the monomers.
 20. Themethod according to claim 7, wherein the free-radical chain initiator isintroduced into a region of the tubular reactor in which the flowvelocity of the mobile flow medium has been increased between 1.8 and2.5 times.
 21. The apparatus as claimed in claim 8, wherein the tubularreactor section has an internal diameter D and a length in the rangefrom 35 to 45·D.
 22. The apparatus according to claim 12, wherein thereis a plurality of feed nozzles arranged at the end of the internal tube,viewed in the flow direction, having exit bores of at most 0.7 mm. 23.The apparatus according to claim 12, wherein there is a plurality offeed nozzles arranged at the end of the internal tube, viewed in theflow direction, having exit bores of at most 0.5mm.
 24. The apparatusaccording to claim 13, wherein a conical transition piece, in the regionof which the internal diameter of the tubular reactor is reduced from Dto between about 0.9 and 0.6·D, is arranged upstream of the separatingelements for separating the mobile flow medium into volume elementsflowing separately from one another or downstream of the feed nozzle(s),and in that the conical transition piece is arranged at a separation ofat most 0.5·D, from the separating elements or the feed nozzle(s) andhas a length in the range from 4 to 6·D.
 25. The apparatus according toclaim 14, wherein the reaction tube, which has a length in the rangefrom 20 to 27·D.